1. Field of the Invention
This invention generally relates to a semiconductor photo detecting element, a manufacturing method of the same, and a manufacturing method of an optoelectronic integrated circuit.
2. Description of the Related Art
An optoelectronic integrated circuit has a structure in which a photo detecting element such as a PIN-type photo diode (PIN-PD) and an electronic element such as a hetero junction bipolar transistor (HBT) are integrated monolithically on a same substrate. A PIN-type photo detecting element has mainly a mesa shape in order to be integrated easily with an electronic device element. For example, Japanese Patent Application Publication No. 9-213988 (hereinafter referred to as Document 1) discloses a PIN-type photo detecting element having a structure in which a PIN-type photodiode composed of InGaAs is provided on an InP substrate and a protective layer composed of InP is provided.
The photo detecting element in accordance with Document 1 is effective with respect to a signal light having a long wavelength range of 1260 nm to 1620 nm mainly used in an optical fiber communication system, because the protective layer composed of InP has a wide transparent wavelength range at a long wavelength side. Here, InP has absorption property in a short wavelength range less than 0.92 μm that is band gap energy thereof. Therefore, most of the signal light is absorbed at the InP protective layer, in a communication system using a multi mode fiber and a signal wavelength of 850 nm. As a result, sufficient responsivity property is not obtained. This is because a carrier generated in the InP protective layer may not move toward the photo detecting element because of an energy barrier between the photo detecting element and the InP protective layer and the carrier generated in the InP protective layer may not contribute to responsivity.
Recently, there is a demand for speeding up and advancing (downsizing and reducing a cost of) a short wavelength communication system using a signal of 850 nm. Conventionally, a silicon photo diode is used in a communication system using a signal wavelength of 850 nm.
However, it is difficult to control a thickness of a light-absorbing layer (Si) of the silicon photo diode. Therefore, there is a limitation in the speeding up of the silicon photodiode. It may not be possible to integrate the silicon photodiode with an InP type electronic device achieving speed-up (for example, a hetero bipolar transistor), because each of the materials is different from each other.
InGaAs has an unstable surface because band gap energy of the InGaAs is small. In this case, it is difficult to protect the surface of the InGaAs with a dielectric layer such as SiN. Therefore, there are many trap energy levels of a generated carrier on the surface of InGaAs. In particular, responsivity with respect to a signal light of a short wavelength range having a high absorption coefficient is reduced because of the surface trap.